Differential hoists employing two drums of different diameters to increase the lifting power are well known in the prior art. Such prior art differential hoists, however, either allow loads to swing or under variable loads do not hold position which attributes are undesirable, and for many applications are wholly unacceptable. The present invention overcomes the foregoing difficulties and allows the driven structure to be rigidly and accurately fixed while accommodating widely variable loads. The present invention also employs a differential escapement which may include a differential pulley or drum having two regions of different diameters. In the present invention, however, the differential escapement means is disposed in combination with a pair of direction turning members, all drivingly interconnected by a flexible force transmitting element, such as a chain, flexible cable, strap or the like arranged in an endless loop to provide a new and improved differential drive or hoist where, through pretensioning, the position of a variable load can be rigidly fixed and restrained from swinging or other unintended movement.
While the invention may be employed as a hoist to lift various types of heavy loads, or to otherwise linearly move or rotatably drive various types of structures, it is especially advantageous for use in driving large outdoor structures such as radiotelescopes, parabolic dishes, and solar radiation collector systems which in order to properly follow satellites and other celestial bodies must be accurately pointed and moved with precision even under the most severe outdoor weather and wind conditions. Accordingly, the invention will also be particularly described in connection with driving such outdoor tracking structures.
For example, this invention is particularly advantageous in providing the driving means for large tracking structures and systems employing a curved rim or arcuate member such as the solar radiation collector systems described and claimed in the Assignee's concurrently filed U.S. patent application Ser. No. 07/834,165, filed Feb. 11, 1992, entitled "Lightweight Distributed Force, Two-axis Tracking, Solar Radiation Collector Structures", the description of which patent application is incorporated herein by reference. Such large tracking structures and systems must be rotatably supported from the ground for precise rotation with respect to the ground and in a secure manner and be capable of withstanding gale force buffeting winds and the other effects of the outdoor environment. The differential drive of this invention can not only readily provide the desired low cost, precision, large reduction ratio drive for such structures, but also securely hold these structures to their ground supports without requiring additional components.
Large outdoor structures such as radiotelescopes, parabolic dish antennas, and solar collectors which follow satellites and other celestial bodies must be accurately pointed and moved with precision. Even when large tracking structures are balanced (i.e. the center of gravity is on the axis of rotation), the drives for such structures must accommodate large unbalanced intermittent forces such as wind loads and the weight of ice or snow. Although none of the following referenced drive arrangements utilize such a differential drive, nor is any other drive known to be in any way related to the present invention, a brief discussion of prior art drive arrangements is believed to be helpful in showing the many attempts which have been made to provide drives for large tracking structures such as solar radiation collector systems and the like, and why the differential drive of this invention may be so advantageously employed for such applications.
FIG. 6 of U.S. Pat. No. 4,145,021 employs a double pulley system to increase the mechanical advantage. The system is not a differential drive but rather is a block and tackle type arrangement including a cable which effects several passes between two spaced apart tackles. Because of the mechanical advantage achieved from the block and tackle arrangement, this linear device is capable of multiplying the torque of a drive motor by a factor of seven, as illustrated. Since there is limited contact for friction (only two 180 degree wraps of cable shown on the drive wheel) this drive would not be entirely satisfactory for use with large outdoor tracking structures since it would have a tendency to slip in rough wet weather. For this type of drive to move and hold many tons of force and since a large pulley is required for long cable life (the diameter of pulleys used should be 50 times the diameter of the cable), a very high torque slow speed gearmotor would be required. Further, since in the drive shown in U.S. Pat. No. 4,145,021 both ends of the cable are fastened, the cable wears in only one region which is not acceptable for drives which must have long operating life outdoors. The overall speed reduction ratio achievable is quite limited compared to that of the differential drive of the present invention. Moreover, to achieve high ratios, very long cable lengths would be required.
One way to reduce the forces and precision required in drives for large outdoor structures such as radiotelescopes, parabolic dish antennas, and solar collectors is to inlcude a large wheel or rim and operate on its diameter which provides large leverage, such as shown in U.S. Pat. Nos. 4,209,231; 4,870,949, and German Patent No. DE 3418-879-A. This approach becomes expensive when precision components such as gears are utilized as machine elements at this scale (e.g. diameters between 10 and 50 meters).
Direct acting positive engagement flexible machine elements such as roller chain arrangements (e.g. U.S. Pat. No. 787,145), toothed belts or perforated bands (e.g. U.S. Pat. No. 3,987,452), cable with buttons or linked chain (U.S. Pat. Nos. 514,669 and 4,209,231) are less expensive to incorporate than a rigid gear/pinion arrangement but the torque of the drive motor and gear reduction must still be substantial in order to insure reliable operation under worst case conditions. Minimizing the backlash of final reduction gears requires the use of premium components, the addition of a dragging brake, an actuated brake, a separate locking mechanism, or other device for preventing lost motion. Lost motion limits accurate tracking and leads to component damage during extended periods when certain segments of the drive components must continually withstand the force of buffeting winds. Another limiting criterion is the requirement to operate for 30 to 50 years in an outdoor environment with ice, dirt and corrosive action of pollution. In some of these prior art systems provision for lubrication must be provided. The cost of mechanical elements of such systems makes drives with redundant parallel components expensive.
Low cost drives usually utilize friction elements, such as direct contact friction wheels (e.g. U.S. Pat. No. 4,470,050), cable or wire rope (e.g. U.S. Pat. Nos. 509,391; 508,393; 670,916; 4,145,021; 4,147,414; 4,491,388, and 4,870,949), or arrangements employing a metal or composite band/belt instead of accurately machined elements. Since these approaches are direct acting and rely on friction, they can allow inordinate slip in buffeting winds especially when lubricated by rain. To provide adequate friction in the case of cable, either multiple turns around a large diameter pinion or many wraps around smaller wheels have been required (e.g. U.S. Pat. No. 4,333,446, FIG. 2B). The reliance on friction can be eliminated by using two separate cables, one end fastened to the moving structure and drive drum respectively and wrapped in opposite directions (U.S. Pat. Nos. 3,466,119 and 4,312,326).
Because all of the above prior art approaches except for that of U.S. Pat. No. 4,145,021 are direct acting with the same surface speed in the final drive element and the mating element of the tracking structure, a high torque, slow speed gearmotor is required. A separate brake, clamp or latch to lock the tracking structure is also usually required in order to insure survival in gale force winds.
A number of prior art patents (U.S. Pat. Nos. 4,282,529; 4,392,140 and 4,396,919) use the term differential drive to describe the differential action of two drive motors to effect the aiming of the structure in two directions. Such type drives can use an endless loop of cable or chain. These patents, however, differ significantly from the present invention since they utilize the torque of the motors directly without mechanical advantage and since they do not employ a differential member disposed mechanically between a pair of direction turning pulleys, they operate on a different principle.
Accordingly, none of the prior art differential hoists or drive systems of which we are aware employ the unique combination and arrangement of elements as in the present invention to provide a differential hoist and drive system wherein the load is restrained against unintended movement. Moreover, none of the prior art drives of which we are aware have been entirely satisfactory for providing a low cost, reliable drive for large tracking structures and providing for long operating life in an outdoor environment with ice, dirt and the corrosive action of pollution in rain.